Release of BaYMn2O5+Î´ Viewed from

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Remarkable Oxygen Intake/Release of BaYMnO Viewed from High-Temperature Crystal Structure Teruki Motohashi, Taira Takahashi, Makoto Kimura, Yuji Masubuchi, Shinichi Kikkawa, Yoshiki Kubota, Yoji Kobayashi, Hiroshi Kageyama, Masaki Takata, Susumu Kitagawa, and Ryotaro Matsuda J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp511648b • Publication Date (Web): 14 Jan 2015 Downloaded from http://pubs.acs.org on January 19, 2015

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The Journal of Physical Chemistry

Remarkable Oxygen Intake/Release of BaYMn2O5+δδ Viewed from High-Temperature Crystal Structure Teruki Motohashi,*,† Taira Takahashi,† Makoto Kimura,† Yuji Masubuchi,† Shinichi Kikkawa,† Yoshiki Kubota,‡ Yoji Kobayashi,§ Hiroshi Kageyama,§ Masaki Takata,|| Susumu Kitagawa,⊥ and Ryotaro Matsuda⊥ † ‡

Faculty of Engineering, Hokkaido University, Sapporo 060-8628, Japan Department of Physical Science, Graduate School of Science, Osaka Prefecture

University, Osaka 599-8531, Japan §

Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan

||

RIKEN SPring-8 Center, RIKEN, Hyogo 679-5148, Japan

⊥

Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto University, Kyoto 606-8501, Japan

ABSTRACT: Crystal structure of double-perovskite type BaYMn2O5+δ was studied by high-temperature synchrotron X-ray diffraction (SXRD) under precisely controlled oxygen pressure to gain deeper understanding of the remarkable oxygen intake/release capability of this oxide. The in-situ SXRD analysis at 750 °C revealed that this oxide undergoes a distinct structural change upon lowering oxygen pressure, from a slightly oxygen-deficient “δ = 1” phase (BaYMn2O5.89; P(O2) = 103 Pa) to an oxygen-vacancy ordered “δ = 0.5” phase (BaYMn2O5.51; P(O2) = 10 Pa). The BaYMn2O5.89 structure (orthorhombic Cmmm) involves statistical distribution of oxygen vacancies within the yttrium plane. Meanwhile, the BaYMn2O5.51 structure (orthorhombic Icma) contains arrays of pyramidal MnO5 and octahedral MnO6 forming an alternate ordering, which is stabilized by a particular Mn3+ orbital ordering with collective displacements of Y3+ arrays. Thus, the discontinuous change in the oxygen content can be attributed to the structural reconstruction with oxygen/vacancy redistribution accompanied by yttrium displacement organization. 1 ACS Paragon Plus Environment


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Keywords:

oxygen

storage

materials,

double

perovskite

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manganese

oxides,

high-temperature crystal structure, synchrotron X-ray diffraction *Corresponding author. Faculty of Engineering, Hokkaido University, N13, W8, Kita-ku, Sapporo 060-8628, Japan. Tel: +81(0)11 706 6741. Fax: +81(0)11 706 6740. E-mail: [email protected]

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1. INTRODUCTION

Nonstoichiometric oxides with reversible oxygen intake/release capability are called oxygen storage materials (OSMs). Such oxides have recently attracted increased attention because of their potential ability for a precise control of redox reactions.1-13 OSMs are generally characterized with their maximum capacity (= oxygen storage capacity; OSC) as well as the oxygen intake/release reaction kinetics. Enhancements in these parameters are of particular importance to develop novel/better materials for energy and environmental applications. Understanding of the associated key factors is the major scientific challenge in the forefront research on OSMs.14-16

The remarkable oxygen intake/release capability has been highlighted for the manganese

oxide,

BaYMn2O5+δ.7

This

oxide

crystallizes

in

the

so-called

double-perovskite type structure which contains an arrangement of smaller yttrium and larger barium ions in separate layers at the perovskite A-site.17-21 The oxygen site within the yttrium plane is readily filled/unfilled in response to variations in temperature and the surrounding atmosphere, resulting in large oxygen nonstoichiometry ranging 0 ≤ δ ≤ 1. BaYMn2O5+δ can reversibly store/release a large amount of oxygen (> 3.7 wt%) at 500 °C within several minutes upon switching the atmosphere between O2 and 5% H2/95% Ar.7 Thus, various oxygen-related applications such as redox agents and catalysts are promising owing to this unique ability.

The oxygen intake/release behaviors of BaYMn2O5+δ are clearly beyond those of conventional perovskite oxides and thus expressed “remarkable” in terms of the magnitude and sharpness of the processes. This emphasizes the importance of its 3 ACS Paragon Plus Environment


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structural feature involving the layered cationic order of Ba2+ and Y3+. Taking into account the fact that the active oxygen site is located within the yttrium plane, the local environment around yttrium is likely one of the key factors for the remarkable oxygen intake/release characteristics. It also appeared that the actual oxygen intake/release reactions may be more complicated than those expressed by a simple chemical equation: BaYMn2O5 (δ = 0) + 1/2O2 ↔ BaYMn2O6 (δ = 1). In fact, our X-ray diffraction (XRD) study on partially oxidized/reduced products revealed a clear indication of two phase coexistence out of three distinct forms with δ ≈ 0, 0.5 and 1.0 (“O5”, “O5.5”, and “O6” phases, respectively).22 This feature would induce a steep chemical-potential gradient at the grain interface, making a positive impact on the oxygen intake/release kinetics. Nevertheless, the origin underlying the three distinct oxygen contents was unclear, especially the appearance of the intermediate form at δ ≈ 0.5.

These findings motivated us to investigate high-temperature crystal structure of BaYMn2O5+δ for the understanding of the remarkable oxygen intake/release from the structural point of view. While high-temperature crystal structure of this oxide was previously investigated by means of neutron diffraction, the measurements were limited for the fully-oxygenated “O6” form at temperatures up to 500 °C.23 Meanwhile, the thermogravimetric studies evidenced22,24 that the intermediate “O5.5” phase appears when oxygenated “O6” samples are heated at 700 ~ 800 °C in flowing inert gas, whereas the “O6” phase remains stable up to 800 °C under oxygen-rich atmospheres. We thus anticipated that the structural transformation between the “O5.5” and “O6” phases should take place upon varying oxygen pressures (P(O2)) at a fixed temperature. In the present work, high-temperature structural analysis was performed on the oxygen storage material BaYMn2O5+δ employing a synchrotron X-ray diffraction (SXRD) technique. 4 ACS Paragon Plus Environment

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The “O5.5” and “O6” phases were found to be stabilized when the sample was heated at 750 °C under P(O2) ≤ 102 Pa for the former and ≥ 103 Pa for the latter. On the basis of their refined atomic arrangements, the structural features underlying the oxygen intake/release processes were discussed.

2. EXPERIMENTAL SECTION

Samples of BaYMn2O5+δ were synthesized via a citrate precursor route combined with the oxygen-pressure-controlled encapsulation technique.7,21 Y2O3 (99.9%, Wako Pure Chemical; fired at 1000 °C overnight prior to use), Ba(NO3)2 (99.9%, Wako Pure Chemical), and Mn(NO3)2⋅6H2O (99.9%, Wako Pure Chemical) were used as starting materials. Appropriate amounts of these reagents were dissolved in diluted HNO3 (for Y2O3) or Milli-Q water (for Ba(NO3)2 and Mn(NO3)2⋅6H2O) to prepare yttrium (0.2 M), barium (0.2 M), and manganese (0.4 M) nitrate solutions. 10 mL of these solutions were mixed in a crucible in which 0.008 mole of citric acid (98%, Wako Pure Chemical) was subsequently added as a complexing agent. The citrate solution was stirred and heated at 60 ~ 70 °C to promote polymerization. The gelatinous product was prefired in air at 450 °C for 1 h and then at 900 °C for 24 h. The resultant precursor powder was pressed into pellets and placed in an evacuated silica ampule together with an equiamount of FeO powder, which acts as a getter for excess oxygen. The silica ampule was heated at 1100 °C for 24 h, followed by quenching into ice water. The related iron oxide BaYFe2O5+δ with a double-perovskite type structure was similarly synthesized to be used as a reference compound. The sample was fired at 1000 °C in an evacuated silica ampule together with metallic Fe instead of FeO.25 5 ACS Paragon Plus Environment


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The as-synthesized BaYMn2O5+δ product was nearly phase-pure of the fully reduced “O5” form, with trace amounts of secondary phases which were marginally detectable by SXRD. The “O5” product was post-annealed at 500 °C in flowing O2 gas for 3 h to obtain a fully oxidized “O6” product. Oxygen content under various P(O2) was analyzed by means of thermogravimetry (TG; Rigaku TG8120). In this measurement, the weight of the “O6” product (~ 30 mg) was monitored at 750 °C while the oxygen partial pressure was increased and decreased in a stepwise manner between P(O2) = 1 and 105 Pa utilizing mass flow controllers and commercial gas mixtures. The initial oxygen content at 750 °C under P(O2) = 105 Pa was determined by means of iodometric titration. Approximately 30 mg of the sample was dissolved in 3 M HCl solution containing an excess of KI as the reductant. Then, the resultant I2 was titrated with 0.020 M Na2S2O3 solution. The 5+δ value determined from five trials was 6.001(4).

In situ powder XRD measurements were performed using synchrotron radiation at the BL02B2 beamline in a synchrotron radiation facility SPring-8, Japan. A large Debye-Scherrer camera equipped with an imaging plate was used for the data acquisition. The wavelength of the incident beam was 0.0354594 nm, which was calibrated with the lattice constant of a standard CeO2 powder (NIST, a = 0.54111 nm). A sieved powder sample was mounted in a silica capillary (outer diameter = 0.4 mm), which was connected with a gas/vapor pressure control system.26 SXRD measurements were carried out at temperatures between 25 and 750 °C under precisely controlled oxygen pressures ranging 10 ~ 103 Pa.27 Diffraction data were collected at 2θ = 0 ~ 80° with a step interval of 0.01°. The crystal structure was refined by the Rietveld method 6 ACS Paragon Plus Environment

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using RIETAN-FP program28 and visualized with VESTA software.29

3. RESULTS AND DISCUSSION

In the isothermal TG experiment at 750 °C, the sample weight is immediately varied and then saturated when P(O2) is increased/decreased in a stepwise manner, as shown in Figure 1a. Noticeably, large weight variations are observed at 4 ~ 5 × 102 Pa both in the oxygen intake/release processes. In fact, the oxygen content vs. P(O2) plot highlights an abrupt jump between δ ≈ 0.55 and 0.85 in a narrow pressure range (Figure 1b). The abrupt jump appears also in the isothermal TG data at 650, 700, 800, and 850 °C; see Figures S1 and S2 of Supporting Information. This clearly indicates the existence of two thermodynamically stable phases in this P(O2) range. The asymmetric oxygen intake/release behaviors near the critical oxygen pressure suggest that the structural transformation is of the first order with an activation barrier.

It is noteworthy that both of the two phases are slightly oxygen nonstoichiometric. The oxygen-rich phase, which we assigned as “O6”, is apparently subject to oxygen deficiencies down to δ ≈ 0.85. The decrease in oxygen content under diluted oxygen atmospheres was also seen for the oxygen-rich phase of isostructural BaGdMn2O5+δ,30 whereas the amount of oxygen deficiencies (1−δ) is rather larger for our Y-sample than the Gd-sample, implying that the reduction of the former is easier. Meanwhile, the partially reduced phase, that is, the intermediate “O5.5”, seems to contain a small amount of excess oxygen (~0.05 per formula unit). The excess oxygen atoms are likely to occupy the vacancy site of the ideal O5.5 structure. This fact agrees well with the previous report by C. Perca et al.,24 who suggested that their “O5.5” product was slightly 7 ACS Paragon Plus Environment


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oxygen excess with a refined chemical formula of BaYMn2O5.55 ± 0.01.

It is now evident that BaYMn2O5+δ crystallizes either in the “O6” or “O5.5” form depending on P(O2) at a fixed temperature. This characteristic feature enables us to study these distinct forms selectively upon varying P(O2). As demonstrated by SXRD data in Figure 2, a drastic structural transformation indeed takes place at 750 °C when the oxygen pressure is decreased from P(O2) = 103 to 102 ~ 10 Pa. The TG result ensures that the SXRD pattern at P(O2) = 103 Pa (Figure 2a) corresponds to the “O6” phase with oxygen deficiencies (δ = 0.89), while the patterns at P(O2) = 102 and 10 Pa (Figures 2b and 2c) the “O5.5” phase containing excess oxygen (δ = 0.51).

The crystal structure of BaYMn2O5.89 (750 °C, P(O2) = 103 Pa) was refined with SXRD data at 2θ = 4 ~ 40°. Diffraction peaks are reasonably indexed on the basis of an orthorhombic unit cell with lattice constants a, b, c ≈ 2ap, where ap denotes the lattice constant of the cubic perovskite structure. No trace of the two other forms, “O5.5” and “O5”, is detected in the diffraction pattern. Following the previous work by A. J. Williams et al.,23 the structural model with Cmmm symmetry was adopted. It should be noted that this space group is consistent with the group theoretical analysis for layered ordering cations at the perovskite A-site.31 Results of our preliminary investigation on the structural evolution at elevated temperatures are given in Supporting Information.

As a first step, refinement of the Cmmm model was attempted assuming a single atomic site for each cation and five nonequivalent sites for oxygen. The secondary phases, assigned as BaCO3 (orthorhombic Pmcn32), Y2O3 (cubic Ia-333), and Mn3O4 (tetragonal I41/amd34) were included in the refinement. The occupancy factors (g) of the 8 ACS Paragon Plus Environment

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five oxygen sites were constrained to reflect the total amount of oxygen (5+δ = 5.89) as determined by TG. Our preliminary analyses indicated that refinements of the oxygen sites, especially the displacement factors (B) as well as the g factors, were somewhat unstable. In fact, g and B of each site were intimately correlated, and often converged to unrealistic values (see a representative case in Supporting Information). Hence, we set the following additional constraints to avoid this difficulty. (i) Oxygen deficiencies are assumed to exist only in the O1 site within the yttrium plane, while the other four sites are fully occupied. This assumption is supported by the recent theoretical work,15,16 which suggests that the oxygen-vacancy formation energy is smaller for the yttrium layer than that for the manganese and barium planes (by 0.31 and 0.64 eV, respectively). (ii) All the B values at the oxygen sites are set to be equal. It should be emphasized that this constraint condition resulted in negligible influences on the atomic coordination for all the sites.

The Rietveld calculation with this condition led to reasonable atomic parameters, except for the yttrium site at which the B value is unusually large (> 2.5 × 10-2 nm2), despite a large atomic weight of yttrium. The large B value is most likely related to statistical displacement of Y3+ ions. In the BaYMn2O5+δ lattice, yttrium resides in a large cavity as compared to its ionic size. Thus, it may be reasonable to assume that Y3+ ions deviate from the symmetrical position at (x, 0, 1/2) so as to satisfy more preferable Y-O bond lengths. Based on this consideration, a site splitting was applied to yttrium atoms locating at (x, ±y, 1/2) with 50% occupancy, that is, g = 0.5 at either +y or −y in a statistical manner. Figure 3 presents the SXRD data together with the fit result of BaYMn2O5.89, and the resultant crystal structure is visualized in the inset. Atomic parameters and refined details are summarized in Tables 1 and 2. 9 ACS Paragon Plus Environment


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All manganese sites in this structure are equivalent with a bond-valence-sum (BVS) value of +3.39,35 which is in perfect agreement with that expected from the actual oxygen composition (+3.39). The single manganese site with mixed valency is consistent with the metallic behavior of BaYMn2O5+δ (δ ≈ 1) at elevated temperatures, revealed by transport measurements in the previous work.36 As seen in the figure, each MnO6 octahedron is rotated alternately clockwise and anticlockwise about the crystallographic a-axis, making severely distorted oxygen square lattice within the yttrium plane. The large distortion is obviously related to the size mismatch between barium and yttrium ions at the perovskite A-site. Details in the local environment around yttrium will be discussed later.

The diffraction pattern for BaYMn2O5.51, taken at 750 °C under P(O2) = 10 Pa, is fitted well with the orthorhombic Icma model (a, b ≈ 2ap, c ≈ 4ap) that is proposed for a room-temperature (RT) structure of BaYMn2O5.55.24 Noticeably, diffraction peaks of the “O6” form completely disappear upon decreasing P(O2). In our structural model, the O6 site within the yttrium plane is filled by 0.01 per formula unit of excess oxygen (g = 0.02), while the neighboring O5 site is fully occupied. The ideal “O5.5” structure contains alternate ordering of oxygen (O5)/vacancy (O6) arrays, giving rise to two distinct manganese sites: one with square pyramidal coordination (Mn1) and the other with octahedral coordination (Mn2). The SXRD data together with the fit result are shown in Figure 4, and the resultant crystal structure in the inset. Atomic parameters and refined details are summarized in Tables 3 and 4. The BVS calculation gives reasonable valence values: +2.85 and +3.21 for the pyramidal Mn1 and octahedral Mn2, respectively, and thereby the average value of +3.03 in excellent agreement with the 10 ACS Paragon Plus Environment

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nominal valence of +3.01.

It has thus appeared that the “O6”-to-“O5.5” transformation involves drastic modifications in the Mn-O polyhedra. Moreover, a closer look at the coordination geometry around manganese has evidenced important structural features underlying the discontinuous change in the oxygen content. Figure 5 illustrates schematic atomic arrangements in BaYMn2O5.89 (upper left) and BaYMn2O5.51 (upper right) including specific Mn-O bond lengths. It can be seen that both the MnO5/MnO6 polyhedra in the BaYMn2O5.51 structure are significantly distorted. For pyramidal MnO5, the Mn-O bond parallel to the c-axis is much longer than those within the ab plane, while octahedral MnO6 is largely elongated along the a-axis. This feature is in contrast to that of BaYMn2O5.89, where the Mn octahedra are much more isotropic having similar Mn-O bonds for all directions.

Such a local atomic arrangement in BaYMn2O5.51 can be interpreted as a consequence of particular orbital ordering. The d4 electronic configuration of Mn3+ is stabilized in pyramidal coordination, as the dz2 orbital pointing to apexes will decrease in energy by removing one axial ligand from an octahedron. Axially-elongated octahedral coordination is also favorable for this electronic configuration because of the diminished electrostatic repulsion between dz2 electrons and axial ligands, which is widely known as a Jahn-Teller (JT) effect. Taking into account these aspects, the high-temperature structure of BaYMn2O5.51 is assumed to consist of alternate orbital ordering as depicted in Figure 5 (bottom): the pyramidal Mn3+ has a dz2-type orbital along the c-axis, while the octahedral Mn3+ has a dz2-type orbital along the a-axis. Although a similar structural model was previously proposed for a RT structure of 11 ACS Paragon Plus Environment


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BaYMn2O5.55,24 our high-temperature diffraction study revealed for the first time the existence of the orbital ordered state even at 750 °C.

A structural comparison of the “O5.5” phase at RT and 750 °C has evidenced essentially identical coordination geometry for both of the MnO5/MnO6 polyhedra, see Figures 5 (upper right) and 5 (middle right). The degree of JT distortion is defined as tJT ≡ d(Mn-Olong)/d(Mn-Oshort), where d(Mn-Olong) and d(Mn-Oshort) are average bond lengths parallel and perpendicular to the elongated direction, respectively. The tJT values at 750 °C are 1.184 for pyramidal Mn1 and 1.093 for octahedral Mn2, being perfectly coincident with 1.180 for the former and 1.092 for the latter at RT.24 This feature clearly indicates the robustness of the “O5.5” structure with significant thermal stability.

The peculiarity of BaYMn2O5.51 is further emphasized when compared with the reference compound BaYFe2O5.50. The SXRD pattern for this oxide, taken at 750 °C under P(O2) = 10 Pa (exactly the same condition as BaYMn2O5.51), was successfully fitted with the orthorhombic Cmmm model that is essentially identical to that for BaYMn2O5.89. Refinement details are given in Supporting Information. Remarkably, oxygen vacancy ordering is absent in BaYFe2O5.50, where the O1 site within the yttrium plane is half occupied in a statistical manner. As expected, the iron octahedra are nearly isotropic because of spherical d5 electronic configuration of Fe3+ (Figure 5, middle left). The absence of uniaxially elongated octahedra in the oxygen-rich “O6” form (BaYMn2O5.89) can also be interpreted as the suppressed JT effect at each manganese ion having the smaller number of d electrons (~d3.6). It is worth noting that the BaYFe2O5.50 lattice is much larger in volume than the BaYMn2O5.51 lattice, despite the opposite relation in the ionic radii of Fe3+ (64.5 pm) and Mn3+ (65.0 pm).37 The smaller 12 ACS Paragon Plus Environment

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lattice volume of BaYMn2O5.51 is attributable to its rationalized “long-short-long-short” alternate order of O-Mn-O units along the a-axis. Thus, the exceptionally stable structure of BaYMn2O5.51 could be one of the primary factors of the abrupt jump in the oxygen content.

Next, we focus on the local environment around yttrium both in the BaYMn2O5.89 and BaYMn2O5.51 structures. For BaYMn2O5.89, Y3+ ions are located at the split sites (0.255, ±0.019, 1/2) within the distorted oxygen square lattice, as illustrated in Figure 6a. The site splitting leads to random displacements of yttrium along the b-axis as large as ±0.015 nm, which are even larger than ±0.004 nm of the zigzag movement in the perpendicular a-axis direction. Due to the presence of the yttrium displacements, two Y-O distances are effectively shortened to 0.249 and 0.287 nm with respect to 0.259 and 0.297 nm for the original unsplit position at (0.255, 0, 1/2). The former values are indeed closer to 0.250 nm expected from the ideal ionic radii of Y3+(IX) and O2-(VI).37 One should recall that the oxygen sites within the yttrium plane are partially unfilled (g = 0.89), such that nonequivalent distances of shorter Y-O and longer Y-VO may be more favorable (VO denotes an oxygen vacancy). The large yttrium displacement is thus suggested to be a consequence of oxygen vacancy formation at the surrounding oxygen sites. Inevitably, the randomly deficient oxygen site within the yttrium plane should be more or less displaced, but unfortunately, we are unable to detect the disrupted nature of the oxygen site due to its small scattering factor of X-ray.

In the BaYMn2O5.51 structure, on the other hand, oxygen vacancies are aligned perfectly, forming oxygen/vacancy arrays (//b-axis) alternately ordered along the a-axis, see Figure 6b. Arrays of the yttrium ions are accordingly displaced towards their 13 ACS Paragon Plus Environment


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adjacent oxygen arrays, with shorter Y-O and longer Y-VO distances. It should be emphasized that this collective displacement of yttrium is intimately linked to the aforementioned Mn3+ orbital ordering. In terms of the atomic arrangement within the yttrium plane, the abrupt “O6”-to-“O5.5” transformation can be viewed as discontinuous structural reconstruction, involving oxygen/vacancy redistribution and yttrium displacement organization, being triggered by the relative stabilization of the two phases. That is to say, the single-step oxygen release of BaYMn2O5+δ under lowered oxygen pressure is attributed to the exceptional stability of the “O5.5” structure consisting of particular Mn3+ orbital ordering, and the instability of the “O6” structure disrupted by the increased number of oxygen vacancies and associated random displacements of yttrium.

4. CONCLUSIONS

The remarkable oxygen intake/release behaviors of double-perovskite type BaYMn2O5+δ were studied from the high-temperature structural point of view. Our in-situ synchrotron X-ray diffraction revealed that this oxide crystallizes in two distinct forms at 750 °C depending on the oxygen pressure: either a slightly oxygen-deficient “O6” phase (BaYMn2O5.89; orthorhombic Cmmm) at P(O2) = 103 Pa, or an oxygen-vacancy ordered “O5.5” phase (BaYMn2O5.51; orthorhombic Icma) at P(O2) = 10 Pa. The BaYMn2O5.89 structure involves statistical distribution of oxygen vacancies within the yttrium plane. Meanwhile, the BaYMn2O5.51 structure contains arrays of pyramidal MnO5 and octahedral MnO6 forming an alternate ordering, which is stabilized by a particular Mn3+ orbital ordering with collective displacements of Y3+ 14 ACS Paragon Plus Environment

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arrays. Thus, the remarkable oxygen intake/release can be attributed to discontinuous structural reconstruction, accompanied by oxygen/vacancy redistribution and yttrium displacement organization.

The present work has highlighted the importance of electronic aspects for tailoring potential OSMs with excellent characteristics. We suggest that manganese is most noteworthy as a redox species, since many Mn-based compounds indeed tend to show abrupt changes in their oxygen contents. Such drastic changes in the oxygen content are often triggered by modifications in the local environments around JT-active Mn3+ and JT-inactive Mn2+/Mn4+, as exemplified by the present BaYMn2O5+δ, and also Brownmillerite-type Ca2AlMnO5+δ (0 ≤ δ ≤ 0.5).10

ACKNOWLEDGMENTS The present work was supported by Grants-in-Aid for Science Research (Contract Nos. 22750181 and 26288104) from Japan Society for the Promotion of Science. T.M. acknowledges financial supports from Iketani Science and Technology Foundation. The synchrotron radiation experiments were performed on the BL02B2 beamline at SPring-8 under the approval of the Japan Synchrotron Radiation Research Institute (JASRI), Proposal No. 2012B1479. We express our sincere gratitude to Dr. J. Kim of JASRI for her help in the SXRD experiment.

ASSOCIATED CONTENT Supporting Information Available (1) Isothermal TG data at 650, 700, 800, and 850 °C, (2) the result of preliminary 15 ACS Paragon Plus Environment


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SXRD measurements at elevated temperatures, (3) Rietveld analyses for BaYMn2O5.89 and BaYMn2O5.51 on the basis of more delicate structural models, (4) Rietveld refinement details for BaYFe2O5.50, and (5) Structural data of the BaYMn2O5.89 and BaYMn2O5.51 phases (in CIF format). This information is available free of charge via the Internet at http://pubs.acs.org


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Kadota, S.; Karppinen, M.; Motohashi, T.; Yamauchi, H. R-Site Substitution

Effect on the Oxygen-Storage Capability of RBaCo4O7+δ. Chem. Mater. 2008, 20, 6378-6381. (6) Singh, P.; Hegde, M.S.; Gopalakrishnan, J. Ce2/3Cr1/3O2+y: A New Oxygen Storage Material Based on the Fluorite Structure. Chem. Mater. 2008, 20, 7268-7273. (7) Motohashi, T.; Ueda, T.; Masubuchi, Y.; Takiguchi, M.; Setoyama, T.; Oshima, K.; Kikkawa, S. Remarkable Oxygen Intake/Release Capability of BaYMn2O5+δ: Applications to Oxygen Storage Technologies. Chem. Mater. 2010, 22, 3192-3196. (8) Motohashi, T.; Ueda, T.; Masubuchi, Y.; Kikkawa, S. Enhanced Oxygen Intake/Release Kinetics of BaYMn2O5+δ Fine Powders Prepared by A Wet-Chemical Route. J. Ceram. Soc. Jpn. 2011, 119, 894-897. (9) Remsen, S.; Dabrowski, B. Synthesis and Oxygen Storage Capabilities of Hexagonal Dy1-xYxMnO3+δ. Chem. Mater. 2011, 23, 3818-3827. (10) Motohashi, T.; Hirano, Y.; Masubuchi, Y.; Oshima, K.; Setoyama, T.; Kikkawa, S. Oxygen Storage Capability of Brownmillerite-type Ca2AlMnO5+δ and Its Application to Oxygen Enrichment. Chem. Mater. 2013, 25, 372-377. (11) Swierczek, K.; Klimkowicz, A.; Zheng, K.; Dabrowski, B. Synthesis, Crystal Structure and Electrical Properties of A-Site Cation Ordered BaErMn2O5 and BaErMn2O6. J. Solid State Chem. 2013, 203, 68-73. (12) Ran, R.; Wu, X.; Weng, D.; Fan, J. Oxygen Storage Capacity and Structural Properties of Ni-Doped LaMnO3 Perovskites. J. Alloys Compd. 2013, 577, 288-294. (13) Hervieu, M.; Guesdon, A.; Bourgeois, J.; Elkïm, E.; Poienar, M.; Damay, F.; Rouquette, J.; Maignan, A.; Martin, C. Oxygen Storage Capacity and Structural 17 ACS Paragon Plus Environment


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Flexibility of LuFe2O4+x (0 ≤ x ≤ 0.5). Nat. Mater. 2014, 13, 74-80. (14) Parkkima, O.; Yamauchi, H.; Karppinen, M. Oxygen Storage Capacity and Phase Stability of Variously Substituted YBaCo4O7+δ. Chem. Mater. 2013, 25, 599-604. (15) Gilleßen, M.; Lumeij, M.; George, J.; Stoffel, R.; Motohashi, T.; Kikkawa, S.; Dronskowski, R. Oxygen-Storage Materials BaYMn2O5+δ from the Quantum-Chemical Point of View. Chem. Mater. 2012, 24, 1910-1916. (16) Gilleßen, M.; Lumeij, M.; George, J.; Stoffel, R.; Motohashi, T.; Kikkawa, S.; Dronskowski, R. Oxygen-Storage Materials BaYMn2O5+δ from the Quantum-Chemical Point of View (Addition/Correction). Chem. Mater. 2013, 25, 4460. (17) Chapman, J. P.; Attfield, J. P.; Molgg, M.; Friend, C. M.; Beales, T. P. A Ferrimagnetic Manganese Oxide with a Layered Perovskite Structure. Angew. Chem. Int. Ed. 1996, 35, 2482-2484. (18) Millange, F.; Suard, E.; Caignaert, V.; Raveau, B. YBaMn2O5: Crystal and Magnetic Structure Reinvestigation. Mater. Res. Bull. 1999, 34, 1-9. (19) Nakajima, T.; Kageyama, H.; Ueda, Y. Successive Phase Transitions in a Metal-Ordered Manganite Perovskite YBaMn2O6. J. Phys. Chem. Solids 2002, 63, 913-916. (20) Kageyama, H.; Nakajima, T.; Ichihara, M.; Ueda, Y.; Yoshizawa, H.; Ohoyama, K. New Stacking Variations of the Charge and Orbital Ordering in the Metal-Ordered Manganite YBaMn2O6. J. Phys. Soc. Jpn. 2003, 72, 241-244. (21) Karppinen, M.; Okamoto, H.; Fjellvåg, H.; Motohashi, T.; Yamauchi, H. Oxygen and Cation Ordered Perovskite, Ba2Y2Mn4O11. J. Solid State Chem. 2004, 177, 2122-2128. (22) Motohashi, T.; Ueda, T.; Masubuchi, Y.; Kikkawa, S. Oxygen Intake/Release Mechanism of Double-Perovskite Type BaYMn2O5+δ (0 ≤ δ ≤ 1). J. Phys. Chem. C 2013, 117, 12560-12566. (23) Williams, A.; Attfield, J. P.; Redfern, S. A. High-Temperature Orbital, Charge, and Structural Phase Transitions in the Cation-Ordered Manganites TbBaMn2O6 and YBaMn2O6. Ferro-Orbital Order in the Charge- and Cation-Ordered Manganite YBaMn2O6. Phys. Rev. B 2005, 72, 184426/1-184426/13. (24) Perca, C.; Pinsard-Gaudart, L.; Daoud-Aladine, A.; Fernández-Díaz, M. T.; Rodríguez-Carvajal, J. Crystal and Magnetic Structures of the Mn3+ Orbital Ordered Manganite YBaMn2O5.5.Chem. Mater. 2005, 17, 1835-1843. (25) Nakamura, J.; Lindén, J.; Suematsu, H.; Karppinen, M.; Yamauchi, H. Iron Mixed-Valence Compounds, BaSm(Cu0.5+xFe0.5-x)2O5+δ I: Synthesis and Chemical Characterization. Physica C 2000, 338, 121-125. (26) Kato, K.; Hirose, R.; Takemoto, M.; Ha, S.; Kim, J.; Higuchi, M.; Matsuda, R.; 18 ACS Paragon Plus Environment

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Kitagawa, S.; Takata, M. The RIKEN Materials Science Beamline at SPring-8: Towards Visualization of Electrostatic Interaction. AIP Conf. Proc. 2010, 1234, 875-878. (27) Kubota, Y.; Takata, M.; Kobayashi, T. C.; Kitagawa, S. Observation of Gas Molecules Adsorbed in the Nanochannels of Porous Coordination Polymers by the In Situ Synchrotron Powder Diffraction Experiment and the MEM/Rietveld Charge Density Analysis. Coord. Chem. Rev. 2007, 251, 2510. (28) Izumi, F.; Momma, K. Three-Dimensional Visualization in Powder Diffraction. Solid State Phenom. 2007, 130, 15-20. (29) Momma, K.; Izumi, F. VESTA 3 for Three-Dimensional Visualization of Crystal, Volumetric and Morphology Data. J. Appl. Crystallogr. 2011, 44, 1272. (30) Taskin, A. A.; Lavrov, A. N.; Ando, Y. Achieving Fast Oxygen Diffusion in Perovskites by Cation Ordering. Appl. Phys. Lett. 2005, 86, 091910/1-091910/3. (31) Howard, C. J.; Zhang, Z. Structures and Phase Transition in the Layered Perovskite La0.6Sr0.1TiO3: A New Orthorhombic Structure Solved from High-Resolution Diffraction in Combination with Group Theoretical Analysis. J. Phys.: Condens. Matter 2003, 15, 4543-4553. (32) Antao, S. M.; Hassan, I. BaCO3: High-Temperature Crystal Structures and the Pmcn → R3m Phase Transition at 811°C. Phys. Chem. Minerals 2007, 34, 537-580. (33) O’Connor, B. H.; Valentine, T. M. A Neutron Diffraction Study of the Crystal Structure of the C-Form of Yttrium Sesquioxide. Acta Cryst. 1965, B25, 2140-2144. (34) Jarosch, D. Crystal Structure Refinement and Reflectance Measurements of Hausmannite, Mn3O4. Mineralogy and Petrology 1987, 37, 15-23. (35) Brown, I. D.; Altermatt, D. Bond-Valence Parameters Obtained from a Systematic Analysis of the Inorganic Crystal Structure Database. Acta Cryst. 1985, B41, 244-247. (36) Nakajima, T.; Kageyama, H.; Ichihara, M.; Ohoyama, K.; Yoshizawa, H.; Ueda, Y. Anomalous Octahedral Distortion and Multiple Phase Transitions in the Metal-Ordered Manganite YBaMn2O6. J. Solid State Chem. 2004, 177, 987-999. (37) Shannon, R. D.; Prewitt, C. T. Effective Ionic Radii in Oxides and Fluorides. Acta Cryst. 1969, B25, 925-946.



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Table 1. Atomic parameters for BaYMn2O5.89 at 750 °C under P(O2) = 103 Pa. B / 10-2 nm2 atom site g x y z Y

8q

0.5

0.2553(6)

0.0187(6)

1/2

1.62(5)

Ba Mn O1

4i 8o 4h

1 1 0.89

0.2501(6) 1/2 1/2

0 0.2492(7) 0.2923(18)

0 0.2582(2)

1.24(2) 1.02(2)

O2

4g

1

1/2

0.2417(48)

1/2 0

2.34(7) 2.34(7)

O3 O4 O5

4l 4k 8m

1 1 1

0 0 1/4

1/2 0 1/4

0.2779(57) 0.2654(51) 0.2981(15)

2.34(7) 2.34(7) 2.34(7)


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Table 2. Structural refinement details for BaYMn2O5.89 at 750 °C under P(O2) = 103 Pa. BaYMn2O5.89 crystal system

orthorhombic

space group a / nm b / nm c / nm unit cell volume / nm3

Cmmm 0.78323(2) 0.78171(2) 0.77518(2) 0.47461(2) 4

Z dMn-O1 / nm dMn-O2 / nm dMn-O3 / nm

0.1904(4) 0.2002(3) 0.1954(7)

dMn-O4 / nm

0.1961(6)

dMn-O5 / nm BVS for Mn secondary phases

0.1982(2) +3.39 BaCO3 (0.78 wt%) Y2O3 (0.63 wt%) Mn3O4 (0.50 wt%) 2.09% 2.43% 2.82% 2.43

RB RF Rwp S



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Table 3. Atomic parameters for BaYMn2O5.51 at 750 °C under P(O2) = 10 Pa. B / 10-2 nm2 atom site g x y z Y

8j

1

0.2726(2)

Ba Mn1 Mn2 O1 O2

8j 8f 8f 16k 8f

1 1 1 1 1

O3 O4 O5 O6

8j 8j 4b 4a

1 1 1 0.02

0

0.0032(3)

1.43(4)

0.2497(9) 0 0 1/4 0 1/4 0.2296(10) 0.2789(10)

0.2515(2) 0.1156(2) 0.3763(3) 0.0985(3)

1.24(3) 1.09(7) 1.02(7) 0.92(9)

0 0.0420(16) 0.9726(16) 0 0

0.2535(10) 0.0892(7) 0.3839(9) 1/2 0

0.92(9) 0.92(9) 0.92(9) 0.92(9) 0.92(9)

1/4 0 0 1/4 1/4


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Table 4. Structural refinement details for BaYMn2O5.51 at 750 °C under P(O2) = 10 Pa. BaYMn2O5.51 crystal system

orthorhombic

space group a / nm b / nm c / nm unit cell volume / nm3

Icma 0.81885(3) 0.75957(3) 1.53903(6) 0.95724(6) 8

Z dMn1-O1 / nm dMn1-O2 / nm dMn1-O3 / nm

0.1911(9) 0.2123(17) 0.1972(4)

dMn1-O6 / nm

0.1778(4)

BVS for Mn1 dMn2-O1 / nm dMn2-O2 / nm dMn2-O4 / nm dMn2-O5 / nm BVS for Mn2 secondary phases

+2.85 0.2258(9) 0.1890(17) 0.1916(2) 0.1904(5) +3.21 BaCO3 (0.73 wt%) Y2O3 (0.65 wt%) Mn3O4 (0.59 wt%) 1.49%

RB RF Rwp

1.76% 3.38%

S

3.87



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Figure captions Figure 1. (a) Isothermal TG curves (750 °C) for BaYMn2O5+δ under increased/decreased oxygen pressures ranging P(O2) = 1 ~ 105 Pa. (b) The saturated 5+δ value vs. P(O2) plot based on the TG data. Figure 2. Synchrotron X-ray diffraction (SXRD) data of BaYMn2O5+δ measured at 750 °C under P(O2) = (a) 103, (b) 102, and (c) 10 Pa. To highlight the distinct structural change upon lowering oxygen pressure, the patterns in a limited region of 2θ = 5 ~ 10° are presented. Figure 3. Rietveld refinement of the SXRD pattern for BaYMn2O5.89 measured at 750 °C under P(O2) = 103 Pa. Tick marks below the pattern denote the positions of the Bragg peaks for BaYMn2O5.89 (first row), BaCO3 (second row), Y2O3 (third row), and Mn3O4 (fourth row). The resultant crystal structure is visualized in the inset (for simplicity, the yttrium displacement is omitted). Figure 4. Rietveld refinement of the SXRD pattern for BaYMn2O5.51 measured at 750 °C under P(O2) = 10 Pa. Tick marks below the pattern denote the positions of the Bragg peaks for BaYMn2O5.51 (first row), BaCO3 (second row), Y2O3 (third row), and Mn3O4 (fourth row). The resultant crystal structure is visualized in the inset. Figure 5. Atomic arrangements of: BaYMn2O5.89 at 750 °C (upper left), BaYMn2O5.51 at 750 °C (upper right), BaYFe2O5.50 at 750 °C (middle left), and BaYMn2O5.55 at RT (middle right). The bond lengths of BaYMn2O5.55 at RT are calculated from the literature data.24 For simplicity, the O6 site with negligible occupancy is omitted in the illustrations of BaYMn2O5.51 (750 °C) and BaYMn2O5.55 (RT). The schematic illustration of the Mn3+ orbital ordering is given at the bottom. Figure 6. [001] projections of the BaYMn2O5.89 (a) and BaYMn2O5.51 (b) structures, showing local atomic arrangements within the yttrium plane. In this figure, yttrium and oxygen atoms are represented with green and red spheres, respectively. 24 ACS Paragon Plus Environment

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Intensity (a. u.)

400 107

321 2

311 131

222

103 Pa

221 003 Mn3O4

Y2O3

201, 021

(a)

022

10 Pa

220, 202

(b)

5

224

024

10 Pa

222 006 Mn3O4 312 303

105

114 Y2O3

022

004 020 BaCO 3 202

(c)

200, 020 002 BaCO 3

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204,220


6 7 8 9 2θ (λ = 0.0354 nm) / deg.

10

Figure 2. Motohashi et al.


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